After fighting neglected diseases in Africa for a quarter century, former president Jimmy Carter takes on one of the continent's biggest killers malaria

Those rivers were part of the problem. Day after day, I saw women doing their wash in the swift current, where boys gathered water in goatskins and men watched over cattle slurping from the river. The same waters were home to a black fly of the Simulium species, a nasty little customer that cut a wide swath in Ethiopia, infecting more than 3 million people with onchocerciasis, or river blindness, and placing some 7.3 million at risk for the disease. The flies feed on human blood. In the process, they infect their hosts with the parasite Onchocerca volvulus, which gives the disease its scientific name. Once inside a person, the worm-like parasites live for up to 15 years, producing millions of little worms called microfilariae. These crawl under the skin, causing inflammation, lesions, intense itching and a mottling of the epidermis known as "leopard skin." In severe or prolonged cases, they may migrate to the eyes, causing impaired vision or blindness.

By the time 78-year-old Mekonen Leka turned up at the Afeta health clinic in February, he was complaining of blurred vision and intense discomfort. His bony shins and ankles were spotted with the white blotches characteristic of onchocerciasis, and long, livid scars ran up and down his legs from constant scratching. "I feel like there's something crawling around under my skin," he said, reaching for a sharp stick to dig at his shins once more. He sat propped against a tree, his legs splayed before him, telling his story: he was infected six years ago, while planting coffee near the river; the itching kept him from sleeping; that, in turn, kept him from working; one of his children took care of him; he could still see well enough to get around, but he worried that his eyesight would get worse.

"It makes me very weak," said Mekonen. "I have come to the clinic to beg for medication to stop the itching," he said.

The medication was ivermectin, a drug originally developed for deworming cattle and dogs and now produced for people as Mectizan by Merck & Co., which donates the doses. In 2006 alone, the Carter Center distributed more than 2.5 million treatments to Ethiopians. Administered once a year in tablet form, the drug does not kill the adult worms, but it controls the spread of their offspring, which gives patients like Mekonen immediate relief from itching. More important, it will ensure that his vision would deteriorate no further.

He would have to wait a few weeks for the drug, when area health clinics planned a massive treatment. I witnessed several of these drug campaigns, which were run and attended by Africans. The events were announced in local markets and promoted on posters, resulting in hundreds of patients showing up for medicine on a specified day. Nobody just pocketed his pills and went home, but swallowed them under the watchful eye of health workers who dutifully recorded each patient's name in a bound ledger, measured his or her height with a wooden stick, and determined the proper dosage accordingly.

"Our number one complaint is still malaria," said Yeshimebet Moges, the chief nurse at the Afeta health clinic, where patients were lined up 30 deep for afternoon checkups. "But we're seeing a lot of onchocerciasis patients too. They itch; they get the treatment; the complaints disappear. And there's a side benefit—a lot of people have discovered that the medicine also gets rid of intestinal worms and other parasites, which can be a big problem for children. Our patients are really grateful—they bring us coffee, bananas, avocados, oranges." A few satisfied customers have even named their children Mectizan, a tribute to the drug that healed them.

Mectizan means that millions of children will not go blind or know the misery that onchocerciasis has visited on their elders. With luck, a new generation might be well enough to attend school instead of suffering at home or staying there to nurse a sick parent. "If one family member is taken out of production because of sickness, it affects the whole family and eventually the economic development of Ethiopia," said Teshome Gebre. "This is one of the reasons we have a poor country. You need healthy working people if you want to reach the dream of economic progress."

It had been a long while since Sintayhu Tegegn, 45, felt well enough to work. The widowed mother of three sons had contracted trachoma, a bacterial infection occurring throughout Ethiopia, and now she suffered from trichiasis, a late stage of the disease that affects a million Ethiopians and requires immediate surgery to preserve sight; another 15 million to 20 million have an early phase of the ailment, which can usually be arrested by antibiotics.

"My eyes started hurting two years ago," she told me, as we sat outside a village clinic in Merawi, a dot on the map in northwestern Ethiopia. Along with several hundred patients, she was awaiting eye surgery. "The pain has become unbearable," she said, tightening a white cotton shawl around her head to screen out the sunlight. "I have trouble cooking for my family because the smoke irritates my eyes. Sometimes I can find a neighbor to help me—sometimes I just have to go to the kitchen and endure the pain because there's nobody else to feed my family. I can't sleep because of the pain. I can't close my eyes. If I do, then I can't open them because of the pain." She had them closed as we spoke.

The disease, known from ancient times, is transmitted by flies or spread from person to person. The vector, a fly known as Musca sorbens (cousin to our familiar housefly, good old Musca domestica), swarms around the eyes and runny noses of children, feeding on the moisture and leaving behind a microorganism called Chlamydia trachomatis. Infections from the microorganism settle under the eyelids, which become inflamed and thickened. With time the lids begin to scar from the inside, causing them to curl in upon themselves. When this happens, inverted eyelashes begin to scratch the cornea, which eventually clouds over like frosted glass, causing blindness. In Ethiopia, where the disease is known as "hair in the eye," the ailment afflicts primarily children and the women who care for them.